The developmental regulation of secretory function underlies many import cell biological specializations in higher eukaryotes. For example, the differentiation of resting B cells into highly secretory plasma cells is accompanied by the proliferation of a specialized secretory system. The long-term goal of this proposal is to further our understanding of the developmental regulation of secretory function. The physiological and developmental regulation of secretory function will be studied in the simple plant Arabidopsis. As a model system the expression and function of YPT GTPases will be examined during seedling development, focusing on two processes which are differentially regulated by physiological signaling; inhibition of hypocotyl (stem) elongation by light and the activation of cell elongation during root development. Pursuant to preliminary results, coordinate regulation of YPT secretory gene transcripts will be analyzed. The role of secretory vesicle trafficking in plant development will be further determined through the use of conditional dominant negative mutations in YPT GTPases. Finally, techniques with which to assay the structure and function of the plant secretory pathway will be developed.